1. Field of the Invention
The present invention relates generally to measurement of the thermal shrinkage properties of yarn, more particularly to a process and apparatus for measuring the thermal shrinkage properties such as the thermal shrinkage or the thermal shrinkage stress of yarn, especially, thin or fine yarn less than 300 denier. The measuring process and apparatus may be applied to various kinds of yarn, such as fully drawn yarn, undrawn yarn, partially oriented yarn, textured yarn, spun yarn and fancy yarn of synthetic and natural yarn.
2. Description of the Prior Art
A typical process and apparatus for measuring the thermal shrinkage properties of yarn, for example, the thermal crimp of a textured yarn upon being subjected to hot water, is disclosed in U.S. Pat. No. 3,762,220 granted to Gusack et al.
In the measurement, yarn is caused to travel from a drive roll to a pulley, passing through a idler pulley. The idler pulley is disposed in a hot water bath positioned below the level of the drive roll and pulleys. That is, the yarn travels in a V-shaped path. The drive roll is driven at a constant peripheral speed V.sub.1, and the pulley is driven via a magnetic coupling. When the yarn travels through the hot water bath, thermal crimp of the yarn is generated. Therefore, the peripheral speed of the pulley is reduced so as to absorb the thermal crimp of the yarn. Consequently, the thermal crimp C(%) of the yarn is determined by evaluating the equation of C=[(V.sub.1 -V.sub.2)/V.sub.1 ].times.100, where V.sub.2 is the peripheral speed of the pulley.
The conventional measuring process and apparatus, however, is inaccurate when applied to the measurement of the thermal shrinkage or the thermal shrinkage stress of thin or fine yarn. This is because while the yarn travels along the periphery of the idler pulley, it is subjected to large contact friction, which causes a difference in yarn tension between the positions before and after the idler pulley. This difference is increased by the friction in the bearing of the idler pulley.
This difference in yarn tension can be ignored in the measurement of the thermal shrinkage or thermal shrinkage stress of thick yarn, since the thermal stress of thick yarn is extremely large. However, if the yarn is less than 300 denier, particularly less than 100 denier, the thermal stress is very small. In such cases, the above-mentioned difference in yarn tension before and after the idler pulley results in an error in measurement that cannot be ignored. Further, since the difference in yarn tension before and after the idler pulley is changeable, the conventional measuring process and apparatus lack reproducibility.